Field of the Invention
The present invention relates to a method for development of a nonmagnetic one-component toner and to a method of forming fixed images using the development which are applicable to plain paper copying machines, laser printers, plain paper facsimiles, etc. More particularly, it relates to a method for an electrostatic image development and to a method for forming fixed images suitable for a reprography system utilizing a nonmagnetic one-component development in the case where an encapsulated toner whose shell comprises an amorphous polyester as the main component is used as the toner therefor.
Discussion of the Related Art
Conventionally, when images are formed with copying machines, laser beam printers, etc., the Carlson Method has been generally used (U.S. Pat. No. 2,221,776, 2,297,691 and 2,357,809, "Electrophotography," p22-p41, R. M. Shaffert, 1965, The Focal Press).
In the electrophotographic method, after the electrostatic latent image formed on a photoconductor by optical means is developed in a developing process, it is transferred to a recording medium such as a recording paper in a transfer process and then fixed into the final image generally with large amounts of heat and pressure using a fixing roller in a fixing process. In the conventional method for forming fixed images as described above, however, through the processes from the formation of the electrostatic latent image up to the fixing thereof onto the recording medium, the temperature of the heating element of the fixing device has to remain at a very high level and further a high pressure is required.
On the other hand, since both the photoconductor and the developer device have to be maintained at around room temperature, a considerable distance has to be maintained between the fixing portion and the photoconductor or the developer device, which necessitates in making the machine larger. In addition, it is necessary to force the removal of the generated heat from the system. The noise produced by the forced radiation device and the heat dissipated thereby are one of the major factors of environmental disruption in business offices.
Since the fixing process works independently and fixing is usually carried out with a fixing device at such a high temperature of around 200.degree. C. with a nip pressure of not less than 2 kg/cm, as mentioned above, expensive heat-resistant materials such as heat-resistant resins, heat-resistant rubbers, etc. have to be provided in the periphery of the fixing device. Since the fixing is carried out at a high temperature as described above, such problems as curling and jamming of the paper, etc. are likely to take place. In addition, a fixing failure may take place due to heat absorption by the paper, depending upon its thickness.
In addition, when the fixing requires a high temperature, it takes more time to reach the set temperature so that the waiting time for printing cannot be shortened. In such a case, therefore, this method is unsuitable for devices such as a facsimile which requires quick printings. On the other hand, when fixing is carried out at a temperature of not more than 100.degree. C. using the conventional toners, the softening by heat of the resins contained in the toners cannot be expected, so that fixing mainly takes place by the plastic deformation of the resins. Therefore, a large nip pressure of normally not less than 5 kg/cm is necessary, thereby not only making it necessary to use a large-scale fixing device, but also making the fixing strength of the obtained fixed image poor when compared with that of heat-fixing device, and causing such problems as wrinkling of a paper used as a recording medium.
In view of the above, a novel system for development and a matching toner therefor are in demand. In order to meet this demand, the present inventors have developed an encapsulated toner whose shell comprises an amorphous polyester as the main component, and filed a patent application therefor (Japanese Patent Application No. 4-259088). This encapsulated toner is fixable with a low nip pressure at an extremely low temperature, excellent in offset resistance and blocking resistance when the encapsulated toner is used for heat-and-pressure fixing, thereby making it possible to stably form clear visible images free from background for a large number of copying. Further, in order to achieve a long service life of the developer, it has been necessary to effectively charge the developer at a low stress.